High pressure polyethylene reactors are widely used for the polymerization of ethylene, and include autoclave reactors which operate between 1220 to 2000 bar (122 to 200 MPa) and tubular reactors which operate between 2500 and 3100 bar (250 to 310 MPa), both types of reactors, fresh ethylene from an ethylene supply is compressed to reactor pressure by the combination of a first compressor which pressurizes the ethylene to an intermediate pressure on the order of 300 bar (30 MPa), and a second compressor which pressurizes the fresh ethylene together with recycled ethylene from the 300 bar (30 MN) intermediate pressure up to the final reactor pressure. Both types of reactors create a product mixture comprising principally polymer and unreacted monomer. The mixture leaves the reactor through a high pressure let down valve, and then enters a separation system in which unreacted monomer is separated from the polymer and recycled back to the suction of the second compressor. Hence the monomer supplied to the reactor by the second compressor is a combination of feed or “make-up” monomer supplied by the first compressor and recycled monomer from the separation system.
The polymerization of ethylene is an exothermic process which generates heat. Consequently, there is a need to further cool the product mixture after it has left the reactor. In the prior art, particularly in the case of tubular reactors, such cooling has been accomplished by injecting cold ethylene into it as a “quench” prior to the entry of the product mixture into the separation system. The injection of cold ethylene cools the product mixture as it enters the separation system, thereby promoting the phase separation of the product stream into a polymer-rich liquid phase, and a monomer rich off gas. In particular, the attendant dilution of the product stream with an ethylene rich stream promotes the phase separation of ethylene vinyl acetate (EVA) in the separation system by lowering its concentration in the product stream.
Before the cold ethylene can be injected into the flow of product mixture from the reactor, it first must be compressed to a pressure which will allow such injection. In the past, the required flow of cold, compressed ethylene has been diverted out of the flow of make-up ethylene that the first compressor supplies to the second compressor. However, such a flow scheme requires the first compressor to provide the necessary compression to inject the cold ethylene into the flow of product mixture from the reactor.